Ion accelerating and focusing system



Jan. 5, 1954 Filed July 18, 1950 H. P. YOCKEY ION ACCELERATING ANDFOCUSING SYSTEM 2 Sheets-Sheet l 3% mam/M Jan. 5, 1954 p YQCKEY2,665,384

ION ACCELERATING AND FOCUSING SYSTEM Filed July 18, 1950 2 Sheets-Sheet2 m g 4W Patented Jan. 5, 1954 ION ACOELERATING AND FOCUSIN G SYSTEMHubert P. Yockey, Berkeley, Calif., assignor, by mesne assignments, tothe United States of America as represented by the United States AtomicEnergy Commission Application July 18, 1950, Serial No. 174,548

5 Claims.

My invention relates to ion accelerating and focusing systems, and moreparticularly to a system of electrodes for accelerating and focusingions in electromagnetically operated equipment for the separation ofsubstances or isotopes of elements, such as in calutrons for separatingions on a production scale.

In the calutron, it has been the practice to vaporize a charge materialthrough heating, then pass the neutral vapors into an ionizing chamberwhere they are subjected to electron bombardment to produce ions, removethe ions from the chamber through an exit slit and accelerate them intoa magnetic field at substantially right angles thereto, where they arecaused to follow paths whose curvature depends upon their ionic mass,that is, these paths are of generally arcuate configuration having radiiwhich correspond to their respective masses and velocities. These ionsare then collected in receivers located at or near the focal points ofthe ion beams. Magnetic shimming places this focal point at the 180position.

The ionizing source usually takes the form of an electric are adjacentthe exit slit and parallel to the magnetic field of the equipment,between a hot filamentary cathode and a plate anode located at oppositeextremities of the ionizing chamber. This source is comprised ofpositive and negative ions in equilibrium forming a plasma in vaporcontaining the desired type of atom. The system for removing, focusing,and

accelerating the ions from the ionizing chamber into the magnetic fieldgenerally relies on the walls of the exit slit and probably on theplasma boundary as one electrode and includes two additional spacedelectrodes maintained at appro priate differences in potential withrespect to the walls of the exit slit and to each other, with slitstherein for alignment with the exit slit of the ionizing chamber.

The electrodes in the arrangements heretofore used were bulky, and inview of the structural support requirements, the high potentials and theconfigurations employed, it has been customary to space the electrodesat appreciable distances from each other and from the exit slit. Thisspacing has limited the eiiect of the accelerating potentials upon theions in the ionizing chamber,

.and has been an obstacle to increasing ion flow from the exit slit.This also led to excessive f defocusing and loss of ions. It has limitedthe collection at the receivers and in turn the efii ciency of operationof the device.

1 Applicant with a knowledge of these problems in the prior art has foran object of his invention the provision of an ion accelerating andfocusing system having electrodes with complementary tapered faces toalter the electrostatic field and improve the focusing of the ion beam.

Applicant has as another object of his invention the provision of an ionaccelerating and focusing system having electrodes with overlappingportions for bringing them into close proximity with the exit slit of anionzing chamber to reduce defocusing in the accelerating slit region,and in turn increase the ions collected by the receivers.

Applicant has as a further object of his invention the provision of anion accelerating and focusing system having the intermediate electrodeof a series of electrodes with an aperture or slit which is smaller thanthe slits of the other electrodes for increasing the gradient at theexit slit to provide more ions and decrease the defocusing effect of thesystem, thus making available a greater number of such ions forcollection and increasing the efficiency and output of the system.

Applicant has as a still further object of his invention the provisionof an ion accelerating and focusing system having the outer acceleratingelectrode overlapped within the intermediate electrode of the system toreduce defocusing of the ions in the region of deceleration, and toincrease the strength of the intermediate electrode.

Applicant has as a still further object of his invention the provisionof an ion accelerating and focusing system with improved spacing for theelectrodes to reduce the defocusing path of travel of the ions and limitthe spread of the beam and the scattering of ions.

Other objects and advantages of my invention will appear from thefollowing specification and accompanying drawings and the novel featuresthereof will be particularly pointed out in the annexed claims.

In the drawings, Fig. 1 is a detail of a conventional ion acceleratingand focusing system in general use in electromagnetically operated myimproved intermediate electrodes joined together for mounting. Fig. 4 isan elevation view including a revolved section showing a pair ofdefining the exit slit 2 for the ionizing chamber.

This thin plate also serves as the inner electrode and is generallymaintained at a high positive potential, such as at about 40,000 volts.In practice, this electrode and the slit therein'are made by cutting aslot of appropriate size'in a single thin plate of carbon, normally ofabout 3 5 of an inch to 5; of an inch in thickness, which causes suchplate to run at a high temperature in operation because of the ionbombardmen The use of the thin inner electrode 3 thus enables it to .runcomparatively free from crud, a material usually deposited fromtheplasma region on the electrode. Moreover, any crud formed on the thinplate or electrode is usually concentrated on the under side of theplate where it cannot choke off or closethe exit slit 2.

The intermediate electrode is generally thicker than the inn-erelectrode and-has a slit .5 therein for alignment'with the exit slit 2of the ionizing chamber. The walls definingthe slit 5 arerounded .toreduce sparking and provide an opening for the passage of the ions. Thiselectrode is maintained at a high negative potential of around 40,000volts, and serves to provide-a high accelerating potential for removingions from the ionizing chamber through the exit slit 2 and/or forprojecting them into space where they are acted upon by the usualmagnetic field of the calutron. This action tends to push the plasma inthe ionizing chamber back toward the arccenter, forming on the plasma aconcave surface called the'miniscus, and its shape helps to determinethe sharpness of the focus of the beam. Since this electrode with itshigh negative potential tends to spread and/or defocus the beam assoon-as the ions thereof come within the area of its influence, therelatively great spacing which this form or shape of electrodenecessitates requires the ions to move an appreciable distance throughthe field covered by this electrode, thus giving the beam a considerablespread.

The outer electrode is generally of rectangular shape with a slit ithereinaligned with the slits in'the other electrodes referred to above.customarily maintained at the potential of the tank walls and receiverssince the ion paths are .calculatedfor a region in which no electricfield exists. .Theouterelectrode serves toslow down the ions .to avelocity corresponding to thepo- .tential of thatelectrode. .Italsoassists in. focusing the beam. In short, the outer .electrode isgenerallymaintained ground potential which is customarily more positivethan the intermediate electrode, thereby exerting a focusingeffect uponthe spreading beam, but the appreciable spacing of this electrodefromthe intermediate electrode limits its useful effect.

The output of the above conventionalarrangement is to a large extentdependent on the intensity of the electric field in the region of theinitial ion acceleration, that is, the regionof the meniscus. 'Toincrease these field gradients it "has ng been desired .to use closergeometries.

Such geometries, however, requirebetterfocus- It is ing of the ion beamin this region than is obtained with. the electrodes of conventionalshape heretofore used, for otherwise the beam is either too greatlydefocused at the receiver end of the calutron, to give good operation,or is partially blocked by the intermediate electrode, causing highdrains, sparking, and low efficiency. In the improved electrode systemthe electric fields are altered so as to provide better focusing,thereby permitting the closer geometries to be advantageously used.Equipotential lines on Fig. 1 indicates the areas of influence of eachelectrode with the substantially straight lines c-a and 'bb between theelectrodes being the lines of demarcation. The general shape or scope ofthe ion beam is dotted in.

The improved system, shown in Fig. 2, gains its improved focusingpartially from two features, that is, the use of a thin inner electrodewhose surfaces facing the accelerating fields are inclined to the innerelectrode plane, forming 'at'flat truncated V, and theuseof an.intermediateelectrode whose-surfaces opposite-the inner electrode areinclined at .roughly the same angle to :the

plane of the inner electrode as are the inclined outer surfaces of saidinner electrode. These inclined surfaces produce electric forces orgradients which are directed more-toward the center of the slit in theintermediate electrode, and which tend to reduceanyelectrostaticdefocusing that-may occur atthe edge of the'beam. Theinclined surfaces on the inner electrode are-in practice obtainedby'tapering the outer surface thereof toward'the inside edges of theexitslit. In this arrangement, the'center .equipotentialline a'a' is shifted.toward the intermediate electrode, thus reducing the influence ofthat-electrode on the beam and increasing the focusing force Whiledecreasing the defocusing force, since to the right ofthestraight .line,there is a defocusing force'and to the leftof thatlinethere .is afocusingforce. The lengths'of the spaths of travel of the-ions inthedefocusing areaare likewise reduced. These features may be apparent froman examination of the figure :whichdndicates that the lines on eitherside of the straight one are bowed. Lines normal to them have componentswhioh exert a focusing .or a defocusing force on the ions in the regiondepending upon .the direction in which these .lines are bowed.

Accordingly, arrows have beenplaced on the figure togenerally indicatethe action and direction of these forces for different regions inthe-sys- ,nents. The-general shape of the narrower beam produced by thisarrangement is indicated by the dotted lines ofthefigure.

Another factor contributing to improved focusing between the inner andintermediate electrodes is the making of the slit throughthe;intermediate electrode of smaller size than that of the exit slit ofthe ionizing chamber. By reducin the size of this slit in theintermediate electrodeand by bringing the walls of the electrode closerto the center .of theslit, the slit :center more nearly approaches thepotential of theelectrode. .This, in cooperation with the complementarywalls of the inner and intermediate electrodes, insures improvedfocusing and permits the closer spacin of the electrodes for ejectingand increasing the number of ions leaving the ionizing chamber.

An additional feature is the positioning of the outer electrode near theionizing chamber by extending it into the flared outer portion of theintermediate electrode in overlapping relation therewith while notmaking undue sacrifice in the rigidity of the structure and mounting,thus preserving strength for overcoming the considerable attractiveforce introduced by the high diiTerences in potential of the system. Inthisway, the distance, and therefore the time, the ion travels in aregion of the electrostatic field is reduced to a minimum, therebyminimizing the space charge defocusing as much as possible whilepreserving the space clearance which must be maintained to meetelectrical tolerance requirements.

Now referring more in detail to the structural details of Fig. 2, theconventional ionizing chamher i has an exit slit 2' defined by a plateor electrode 3' of carbon or other suitable material, whose outer wallsor faces slope or taper inwardly at an angle of about 15 to the plane ofthe inner electrode as they approach the exit slit. Positioned adjacentinner electrode 3', which forms one wall of the ionizing chamber, is anintermediate electrode 4' having a central portion 8' which projects orflares inwardly toward the inner electrode 3'. The face or surface ofthe flaring portion of the intermediate electrode opposite the innerelectrode extends in a direction and at an angle which is parallel tothe outer face of such inner electrode, that is, it tapers towards theinner electrode at an angle or about 15 to the plane of such electrode.The opening or slit 5' in the electrode 4 is smaller than the exit slit2 of the ionizing chamber I. The body portion 9' thereof issubstantially rec'- tangular in shape and when combined with theinwardly flaring central portion 8', provides a slanting space betweenthe inner and intermediate electrodes and probably contributes to thebowed configuration of the equipotential surfaces or lines therein whichinfluence the focusing, while positioning the supporting portion 9 backfrom the ionizing chamber I at a suificient distance to permit rigidmounting. In addition, this configuration of the intermediate electrodetends to space the body portion 9' thereof back away from the innerelectrode 3 and tends to reduce any vibration which might arise fromattraction due to electrostatic forces.

The outer electrode 6' has a substantially rectangular body l and arelatively long tapered extension H which projects into the flaredcentral portion 8' of the intermediate electrode 4'. The bore or slit 1in the outer electrode is also tapered inwardly toward the exit slit 2.By positioning the extension H of the outer electrode 6 in spacedoverlapping relation with respect to the flared central portion 8' ofthe intermediate electrode 4, it is possible to bring the deceleratingportion of the outer electrode 6 into proximity with the exit slit whilepositioning the body portion Ill thereof remotely from the innerelectrode 3 to facilitate rigid mounting and to reduce the tendency tovibrate in response to changes in the electrostatic field.

As will be seen from Fig. 3, the intermediate electrodes 4' are made upin pairs joined by a central supporting web I which bridges the bodiesof the electrodes. Also in Fig. 4 it will be seen that the outerelectrodes 6' are similarly fabricated, except that they are joinedthrough a pair of brackets or cross members l6, l6 which bridge theelectrodes-at their ends. From the foregoing, and from an examination ofFig. 1 it will be apparent that the electromagnetically operated unitsfor the separation of isotopes of elements are generally operated inpairs, and that the calutron includes at least two of such units withtwo sets of ionizing chambers, two sets of accelerating electrodes, andtwo sets of receivers, but they may be housed in a single vacuum systemand use a single magnetic field. However, the above explanation is notintended as a limitation on this invention, since it is equallyapplicable to a single, as well as a plurality of units, and individualconventional mountings for the electrodes and other elements can beprovided.

Having thus described my invention, I claim:

1. A system of accelerating and focusing electrodes forelectromagnetically operated equipment for the separation of isotopescomprising an inner electrode adapted to the maintained at a highpositive potential and having an opening for the passage of ions, anintermediate electrode spaced from the inner electrode and having anopening aligned with the opening in the inner electrode, saidintermediate electrode being adapted to be maintained at a high negativepotential for accelerating the ions passing through the inner electrode,and an outer electrode positioned in overlapping spaced relation withrespect to the intermediate electrode, said outer electrode having anopening aligned with and of larger size than the opening in saidintermediate electrode for the passage of ions and being adapted to bemaintained at a potential intermediate that of said inner electrode andsaid intermediate electrode for decelerating said ions.

2. A system of accelerating and focusing electrodes forelectromagnetically operated equipment for the separation of isotopescomprising a slotted inner electrode adapted to be maintained at a highpositive potential for the passage of ions, an intermediate electrodehaving an opening therein of smaller size than the slot of said innerelectrode, said intermediate elec trode being spaced from said innerelectrode and adapted to carry a high negative potential foraccelerating the ions, and an outer electrode spaced from saidintermediate electrode and having an opening therein for the passage ofions, said outer electrode being adapted to be maintained at a potentialintermediate of said inner electrode and said intermediate electrode toprovide deceleration.

3. A system of accelerating and focusing electrodes forelectromagnetically operated equipment for the separation of isotopescomprising an inner electrode being adapted to be maintained at highpositive potential having an opening for the passage of ions, said innerelectrode having an outer surface tapering towards said opening, anintermediate electrode having an opening aligned with that in the innerelectrode for the passage of ions, said intermediate electrode beingadapted to be maintained at a high negative potential to accelerate saidions through the opening therein, said intermediate electrode alsohaving a surface slanting in a direction parallel to the outer surfaceof said inner electrode to reduce defocusing, and an outer electrodehaving an opening therein aligned with the opening in: saidintermediate; electipde for the passage; of, ions, said outer" electrodebeing adapted to, be maintained at: a potential intermediate thepotentials of said inner electrode and said outerelectrode fordecelerating said ions.

4 A system of accelerating and focusing electrodes forelectromagnetically operated equipment for the separation of isotopescomprising an inner electrode being adapted to. be maintained at highpositive potential having an opening therein for the passage of ions, acup shaped intermediate electrode having a flaring portion projectingtowards said inner electrode with an openingtherein aligned with theopening in said inner electrode, said intermediate electrode beingadapted to be maintained at a high negative potential for acceleratingsaid ions, and an outer electrode having a. slotted tapered-V extensionfor projecting into the flared portion of said intermediate electrodefor alignment withv the opening therein, said outer electrode beingadapted to be: maintained at a potential intermediate the potential ofsaid. inner electrode to said intermediate electrode to providedeceleration.

5. A system of accelerating and focusing electrodes forelectromagnetically operated equipment for the separation of isotopescomprising an inner electrode being adapted to be maintained at highpositive potential having an opening for the passage of ions, said innerelectrode having an outer surface tapering towards said opening, a cupshaped intermediate electrode having a flaring portion projectingtowards said inner electrode with an opening therein aligned with theopening in said inner electrode and with an outer surface slanting in adirection parallel to the outer surface of said inner electrode, saidintermediate electrode being adapted to be maintained at a highnegative: potential for accelerating the ions from the inner electrodetowards said intermediate electrode, and an outer electrodehaving aslotted tapered extension for projecting into the flared portion of saidintermediate electrode for alignment with the opening therein, saidouter electrode being adapted to be maintained at a potentialintermediate the potential of said inner electrode and said intermediateelectrode to provide deceleration.

HUBERT P. YOCKEY.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,452,893 Bachman Nov. 2, 1948 2,476,005 Thomas July 12, 19492,536,878 Fleming Jan. 2, 1951 2,551,544 Nier et a1. May 1,1951

